The present invention relates to improved polymerization catalyst compositions of the Ziegler-Natta type, procatalysts for use in forming such catalyst compositions, methods of making such catalyst compositions and procatalysts, and to methods of using the catalyst compositions to make an olefin polymer.
Polymers and copolymers of lower α-olefins, particularly, ethylene, propylene and butylene are widely used throughout the world. These polymeric products typically are relatively inexpensive to manufacture, and they exhibit a number of commercially useful properties. When ethylene is polymerized, the process is relatively uncomplicated in that the product type is not influenced by the manner in which the ethylene molecules add to the growing polymeric chain during polymerization. The polymeric product of ethylene does not generally exist in stereoisomeric forms.
When propylene is polymerized, however, the presence of pendant methyl groups on the polymeric chain provides a possibility of several stereo structures, depending on the manner in which propylene molecules add to the growing chain. Most commercial polypropylene results from the stereoregular addition of propylene molecules in an isotactic configuration. Molecules of polymer having a large amount of stereo-irregular propylene units are amorphous, or non-crystalline, and this form is less desirable than the isotactic form. If present in a significant proportion, the non-crystalline polymer must be removed to provide a more desirable, highly crystalline material. Such non-crystalline polymer is often referred to in the art as “atactic”, although in reality it contains both atactic, short run isotactic, short run syndiotactic, and regio-irregular oligomeric molecules.
The degree of tacticity of polypropylene can be measured by determining the percentage of polymer soluble in a solvent, such as xylene. Stereoregular polypropylene polymers having a high percentage of atactic polymer are said to have a high xylene soluble fraction. A higher amount of atactic polymer tends to decrease the crystallinity, and consequently, the stiffness (flexural modulus) of the polymer.
It is known that the amount of atactic polymer formed during a polymerization can be decreased (also known as increasing the stereoselectivity of the polymerization) by using a selectivity control agent (SCA) also referred to as an external electron donor. Suitable SCAs include silane compounds, particularly dicyclopentyl dimethoxysilane (DCPDMS), cyclopentyl trimethoxysilane (CPTMS), or dicyclohexyldimethyoxysilane (DCHDMS) disclosed in U.S. Pat. Nos. 4,990,479, 5,438,110, and elsewhere.
Ziegler-Natta olefin polymerization catalyst compositions typically comprise a solid component containing magnesium, titanium and halide moieties in combination with an internal electron donor (referred to as the “procatalyst”), a substance that is capable of converting the procatalyst to an active polymerization catalyst (referred to as a “cocatalyst”), and (if used to polymerize higher α-olefins to produce tactic polymers) an SCA or external donor. Suitable internal electron donors include aromatic mono- or di-esters or ether derivatives thereof, such as benzoates, phthalates, and C1-4 alkyl ethers thereof. Conventional cocatalysts include aluminum trialkyls, such as triethylaluminum or triisobutylaluminum, and alumoxanes. The cocatalyst may be combined or complexed with some or all of the internal electron donor, selectivity control agent, or both, if desired. Although variations in any of these catalyst components will influence the performance of the resultant catalyst, the component that appears to offer the greatest opportunity for modification to produce greater catalyst activity is the procatalyst.
The literature is rife with disclosures relating to the various known methods of preparing procatalysts. Examples include: U.S. Pat. Nos. 5,247,032, 5,247,031, 5,229,342, 5,153,158, 5,151,399, 5,146,028, 5,106,806, 5,082,907, 5,077,357, 5,066,738, 5,066,737, 5,034,361, 5,028,671, 4,990,479, 4,927,797, 4,829,037, 4,816,433, 4,728,705, 4,548,915, 4,547,476, 4,540,679, 4,472,521, 4,460,701, 4,442,276, and 4,330,649. One preferred method from among the foregoing disclosures is a method of forming the precursor from a mixture of magnesium dialkoxides and titanium alkoxides that are reacted (chlorinated) with titanium tetrachloride in the presence of an alcohol, an aromatic hydroxide compound, and an aromatic solvent, especially chlorobenzene. In this manner, a solid, highly porous precursor is recovered by selective precipitation upon removal of alcohol from the solution. Also among the foregoing disclosures, U.S. Pat. No. 4,829,037, discloses heat treating the procatalyst in the presence or absence of a solvent, as well as washing the resulting procatalyst multiple times with n-heptane between chlorination steps and after chlorination.